Statistical approaches to the discrimination of mantle- and crust-derived low-Cr garnets using major and trace element data
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Diamond exploration focuses on geochemical analysis of indicator minerals that are more abundant than diamond itself. Among such indicators, low-Cr (Cr2O3 < 1 wt%) garnets from mantle eclogites are problematic since they overlap compositionally with many lower-crust-derived garnets also transported by kimberlite. Misclassification of these garnets may create “false positive” mantle signatures and possible misdirection of exploration efforts. Statistical solutions using major elements in low-Cr garnet (Hardman et al. in J Geochem Explor 186:24–35, 2018) provide improved error rates for the discrimination of low-Cr crustal and mantle garnets recovered from kimberlite. In this study we analysed a large suite of garnets (n = 571) from both crustal and mantle settings, already characterised for major elements, for a wide range of trace elements by laser ablation inductively-coupled plasma mass spectrometry and use these new data along with literature data (n = 169) to evaluate the effectiveness of adding trace elements to garnet-based diamond exploration programs. A new garnet classification scheme, initially using a major-element based filter, uses garnet Sr contents and Eu anomalies to help identify low-Cr garnets that are misclassified using major element methods. Combined with existing methods, our new trace element classifiers offer improvement in classification error rates for low-Cr, crustal and mantle garnets to as low as 4.7% for calibration data.
KeywordsDiamond exploration Eclogite Kimberlite Logistic regression Trace elements
We thank Yuri Kinakin for encouragement and support and Diavik Diamond Mines (2012) Inc., Juanita Bellinger (Rio Tinto), De Beers Canada, Peregrine Diamonds Ltd., Ekati Diamond Mine, Shore Gold Inc., and Slava Spetsius (Alrosa) for their support and provision of samples. We thank Nicole Meyer for her suggestions. We thank William L. Griffin, an anonymous reviewer, and guest editor Alan Kobussen for their comments, which greatly improved the quality of this manuscript. This study represents a portion of MFH’s PhD research funded by Diavik Diamond Mines (2012) Inc. and a Natural Sciences and Engineering Research Council of Canada Collaborative Research and Development Grant in partnership with Diavik Diamond Mines (2012) Inc. to TS and DGP (CRDPJ 476392). Additional analytical work was also funded by a Canada Excellence Research Chair award to DGP.
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